Membrane with uniform pore size and shape, and ordered pore distribution are very desirable because of their superior performance compared to conventional membranes in many applications such as filtration, template synthesis, and catalytic reaction. These characteristics of membranes have not yet been satisfactorily achieved by existing membrane fabrication methods including bombardment and wet etching for track-etched membranes, electrochemical etching for anodic alumina membranes, replication of polymer membranes from molding templates, dissolution of embedded particles to yield porous membranes, and the direct photolithographic method. The photolithographic method can pattern photoresist membranes with well-defined and ordered pores on substrates, which is the nature of photolithography. However, in previous photolithographic methods, the membranes cannot be easily detached from substrates, so that additional sacrificial layers are required. Not only are the cost and complexity of the fabrication process increased, but the quality of membranes produced this way is impaired as well when the sacrificial layer is dissolved either in corrosive solution, or by lateral isotropic etching.
Commercially available track-etch membranes (Nuclepore, Poretics, Osmonics and Millipore), prepared by bombardment and wet etching suffer from difficultly to have good control over the morphology and pore size of the pores. Except for the expensive photolithographic method mentioned above, typically porous membranes made using known methods suffer from a poor coefficient of variation (CV), meaning a wide variation in pore size in all direction, slanted walls through the thickness of the membrane, variation in pore diameter from one pore to the other etc., such that these membranes with poor CV cannot be used for precise analytic applications.
Anodic alumina (Whatman Anapore and Anotech Separation) offers a cheaper and morphologically better alternative to track-etch membranes, but involve handling dangerous reagent. In lab, reactive ion etching (RIE) can fabricate freestanding, perforated membranes with three complicated steps: vapor deposition, photolithography and RIE. In contrast, soft lithography provides a simple way that involves spin-coating a thin layer of liquid prepolymer on a substrate that contains microposts. However, the surface tension of the liquid prepolymer around the microposts leads to uneven surface of the membrane.
It would therefore be advantageous to provide a method of fabrication of free standing open pore membranes having a low CV and capable of providing good quality pores with straight parallel walls through the thickness of the membrane as well as pore diameters that are the same across the membrane. It would also be very useful to provide a method of producing porous membranes in which the pore shape and morphology can be carefully controlled and tuned in order to provide shape selective porous membranes.